1. Field of the Invention
The present invention relates to a photoelectric converter in which a photoelectric conversion device is laminated above a signal transfer device and more particularly to improving the sensitivity of such photoelectric converter.
2. Description of Related Art
A photoelectric converter is an apparatus having photoelectric conversion devices. A photoelectric conversion device is a device for converting light energy into electric energy or electric energy into light energy. Among the photoelectric conversion devices, the device for converting light energy (optical signal) into electric energy (electrical signal) includes a photovoltaic device (solar battery) aiming at generating energy and a photo-receiving device for processing signals. The photo-receiving device includes a photo-diode and a photo-transistor utilizing the photovoltaic effect and a photoconductive cell utilizing the photo-conductive effect.
With the advancement of the technology of integrated circuits, an image sensor which is one of photoelectric converter and which is an apparatus in which photo-diodes or photo-conductive cells and signal transfer devices are integrated on one and same device substrate has come to be used widely.
Various measures are taken in the photoelectric conversion device used in the photoelectric converter in order to convert light energy (optical signal) into electric energy (electrical signal) efficiently. For instance, an electrode of the photoelectric conversion device is formed so as to have a texture structure to scatter light within the photoelectric conversion device and to increase a quantity of energy generated by the photoelectric conversion device (solar battery). However, because the scattering of light caused by the texture structure is probabilistic irregular reflection, it cannot be controlled intentionally and it is unable to increase electric energy (electrical signal) uniformly.
What is drawing special attention lately among the photoelectric converters is the image sensor. With the incoming of multi-media, a digital still camera, a cam-coder and the like using the image sensor are now rapidly spreading and the research on the improvement of the efficiency (conversion efficiency) for converting optical signals entering the image sensor into electric signals is being actively conducted. Because the photoelectric conversion devices and the signal transfer devices are disposed on one and same plane of the same device substrate in the image sensor, regions of the signal transfer devices of the image sensor turn out to be dead spaces where no photoelectric conversion can be made. Then, there has been proposed an image sensor in which a micro lens is provided in order to condense optical signals to a region of the photoelectric conversion device in the photoelectric converter.
There has been also proposed a laminated type image sensor as shown in FIG. 2 in which a photoelectric conversion device is laminated on a signal transfer device to allot functions of photoelectric conversion and of signal transfer in the vertical direction. FIG. 2 is a section view of one pixel of the laminated type image sensor. In the laminated type image sensor, the photoelectric conversion device 202 is formed on a substrate on which the signal transfer device 201 is integrated. Because the dead space may be almost eliminated by constructing the micro lenses as shown in FIG. 2, as compared to the image sensor in which the photoelectric conversion device and the signal transfer device are disposed on one and same plane, the area of the photoelectric conversion device of the image sensor may be increased.
Although FIG. 2 shows a MOS image sensor using a top gate type MOS transistor for the signal transfer device 201, the laminated type image sensor may be constructed by using a CCD image sensor in which a MOS capacitor is utilized for the signal transfer device and an image sensor using another signal transfer device.
With the rapid spread of the image sensors, it is demanded to miniaturize and lower the cost of the image sensor further. In fact, the size of the image sensors is becoming small year by year and the size of an optical system which decides the size of the image sensor has reduced from 2/3 inches in the 1980s to 1/4 inches of the present day. Actually, an image sensor for a 1/4 inches camera is now being commercialized. The image sensor may be also fabricated at low cost by miniaturizing the optical system of the image sensor.
To obtain a high image quality image sensor having the same or higher resolution with/over the prior art one while reducing its size, the image sensor must be densified by reducing the size of one pixel. In fact, the size of the pixel is becoming small along the reduction of the size of the optical system of the image sensor and an image sensor having pixels whose size is 10 .mu.m.times.10 .mu.m or less is being commercialized. It is then expected that the reduction of the size of the pixel advances further for the future.
When the size of one pixel of the image sensor is small, optical signals entering one pixel also decrease accordingly. Then, because electric signals converted by the photoelectric conversion device of the image sensor decrease, i.e., electric charges are generated less, the relative rate of noise generated by the electrical effects in and out of the apparatus to the quantity of generated electric charges increases, thus dropping the sensitivity.
As described above, there arises a problem that the sensitivity of the image sensor drops when the size of one pixel of the image sensor is reduced in order to obtain the small and high image quality image sensor. The problem of the drop of the sensitivity of the image sensor which occurs when the size of the pixel is reduced cannot be solved fully just by providing the prior art micro lens or by laminating the photoelectric conversion device with the signal transfer device.
The sensitivity may be enhanced by increasing the quantity of generated electric charges by confining light in a photoelectric conversion layer of the image sensor. However, when the photoelectric conversion device having the electrode in the texture structure is used for the image sensor, scattered light cannot be controlled and the sensitivity disperses among the respective pixels.